CENTERVILLE — Barnstable police responded to a Bent Tree Drive house at about 8 a.m. Thursday after a contractor discovered what appeared to be a grenade in a wall, only to find that the device was a smoke canister.

The M18 smoke grenade, believed to be from the Vietnam War era or early 1980s, was clearly marked and still inside a protective shipping container when police arrived at 8:10 a.m. to find it in the front yard, Barnstable police Sgt. Michael Riley said.

"I believe the people there had some concerns, so they called us. Clearly they didn't know about it, so it was unusual for them," Riley said. "It was pretty clearly marked as 'red smoke.' It's not something you expect when you start poking around in your house."

Smoke grenades are not considered weapons. They are primarily used to signal aircraft or screen troop movements.

Police had the contractor, along with the family living at the house, move away from the canister until officers could arrive to evaluate it.

Barnstable police Sgt. Michael Damery, commander of the Cape Cod Regional Law Enforcement Council SWAT Team, was familiar with the device and secured it without the need for the state police bomb squad. The smoke grenade was taken to the Barnstable Police Department, where it will remain until the state police destroy it, Riley said.

Police will not investigate whether any other items were left inside the house's walls, Riley said, adding that it was unclear why the device was kept there.

"I'm sure the contractor will notify us if he finds anything else," Riley said.

"It's not uncommon for people to call us, especially if they're cleaning out a parent's or grandparent's house, that they find unusual items that someone brought back from their military service," he added. "Inside the wall is a little bit different. But it's not unusual."

In almost all cases, the items are not dangerous, Riley said. But he urged residents to call police if they find a military device.

"Please feel free to call your local police department, but please don't bring it into the police department," he said.